This invention relates to a method and an apparatus for molding a glass product such as an optical lens blank and, in particular, to a method and an apparatus of a so-called direct press system in which a molten glass is supplied to a mold to be pressed in the mold.
As a method of molding an unfinished glass product (hereinafter called a lens blank) as a state before it is ground and polished into a finished or final glass product such as an optical lens, use has widely been made of a direct press system excellent in productivity. In the direct press system, a glass material is melted in a furnace into a molten glass. An appropriate amount of the molten glass is cut off by the use of a cutter such as a shear to be supplied onto a lower die (drag). The molten glass supplied onto the lower die forms a glass gob having a generally marble-like shape under the effect of its surface tension. The lower die with the glass gob placed thereon is transferred by a conveying arrangement such as a turntable to a position where an upper die (cope) is arranged. Then, the glass gob is pressed by the upper and the lower dies to mold a glass product as the above-mentioned lens blank having a shape matching the configuration of a cavity surrounded by the upper and the lower dies. In a grinding/polishing step subsequently carried out, the surface of the lens blank is ground and polished to produce a final product such as a spectacle lens, a camera lens, and an optical pickup lens.
On the other hand, it is recently pointed out that grinding and polishing scraps produced in the grinding/polishing step have an adverse influence upon the environment. Taking this into account, it is strongly desired to mold the lens blank which will require only a small grinding allowance so as to reduce the grinding and polishing scraps.
In the molding of the lens blank by the use of the existing direct press system, however, it is impossible to considerably reduce the grinding allowance of the glass. Specifically, in the existing direct press system, so-called sink marks are distributed on a glass surface due to shrinkage of the glass after the molten glass is pressed. Upon pressing the molten glass, an outer peripheral part of the molten glass is cooled by external air after the molten glass is supplied to the lower die and before it is pressed by the upper and the lower dies. Therefore, the temperature of the molten glass is remarkably lowered in the outer peripheral part as compared with its inner part. If the temperature of the outer peripheral part becomes lower than a certain temperature, the flowability of the glass is decreased so that the viscosity of the glass reaches an inappropriate level at which press-molding is difficult. Taking this into consideration, it is necessary to press the molten glass before the temperature of the inner part of the molten glass is sufficiently cooled, i.e., while the viscosity of the glass is as low as about 103 poises (dPaxc2x7s). This results in an increase in heat shrinkage of the inner part of the glass after pressing and in occurrence of sink marks on the glass surface due to the temperature difference between the inner part and the outer peripheral part. The occurrence of sink marks deteriorates a reproducibility in transferring the shape of a molding surface of the mold so that a large grinding allowance is required. In case where the sink marks produced on the glass surface after pressing are not uniformly distributed but concentrated to a local area, a much larger grinding allowance is inevitably required.
In order to suppress the occurrence of sink marks, various proposals have been made (see Japanese Unexamined Patent Publications (JP-A) Nos. 10-101347, 6-32624, 6-72725, 6-157051, and 63-162539). However, no disclosure is made about a technique for effectively reducing the temperature difference between the inner part and the outer peripheral part of the molten glass, which results in occurrence of sink marks.
On the other hand, in the direct press system, a lower part of the molten glass is continuously cooled towards the temperature of the lower die after it is supplied to the lower die preliminarily kept at or around a glass transition point Tg. Therefore, limitation is imposed such that the molten glass must be pressed and molded before the viscosity of the molten glass at the lower part exceeds a threshold viscosity beyond which the press-molding is impossible.
It is therefore an object of this invention to provide, in molding of a glass in a so-called direct press system, a method and an apparatus capable of molding a glass product which will require a reduced grinding allowance when it is processed into a final glass product.
It is a specific object of this invention to provide a method and an apparatus capable of effectively reducing the heat energy of a molten glass and significantly reducing the temperature difference between inner and outer peripheral parts of the molten glass to effectively suppress occurrence of sink marks on a glass surface after a pressing step.
It is another object of this invention to provide a method and an apparatus for molding a glass product, which are capable of suppressing occurrence of sink marks concentrated to a local area of a glass surface.
It is still another object of this invention to provide a method and an apparatus for molding a glass product, which are capable of suppressing occurrence of sink marks without decreasing a productivity attained in a direct press system.
It is an additional object of this invention to provide a method of producing a glass product capable of reducing grinding and polishing amounts
According to this invention, a method of molding a glass product by pressing a glass gob by the use of a mold composed of an upper die and a lower die, each of the upper and the lower dies having a molding surface, comprises at least the following steps. Specifically, the method comprises a supplying step of supplying as a glass gob a molten glass onto the molding surface of the lower die, a cooling step of cooling an upper surface of the glass gob supplied onto the molding surface of the lower die, a heat radiation suppressing step of suppressing heat radiation from the glass gob so that an inner part and an upper part of the glass gob are close in temperature to each other, and a pressing step of pressing the glass gob by the molding surfaces of the upper and the lower dies when the glass gob has a viscosity within a range between 103.5 and 106.5 poises (dPaxc2x7s). The heat energy of the glass gob supplied onto the lower die is removed in a short time by cooling the upper surface of the glass gob with cooling means, for example, a cooling member to be contacted with the upper surface. Thus, the total heat energy of the glass gob is made to be close to that corresponding to optimum pressing. Furthermore, after the upper surface is cooled in the cooling step so that the temperature difference between the inner part and the upper surface is increased, heat radiation from the glass gob is suppressed in a time period after the cooling step and before the pressing step so that the temperature of the inner part and the temperature of the upper surface are made to approach each other while the total energy of the glass gob is not substantially reduced. Thus, the glass gob as a whole is kept at a relatively high viscosity before the pressing step. By pressing the glass in the above-mentioned state, it is possible to reduce heat shrinkage of the glass after pressing and to reduce the difference in heat shrinkage between the inner and the outer peripheral parts of the glass. Thus, occurrence of sink marks on a glass surface is suppressed.
According to this invention, there is provided a method of producing a final glass product from a glass product molded by pressing a glass gob, the method comprising the steps of molding the glass product by the use of the above-mentioned method and grinding and polishing a surface of the glass product to produce the final glass product.
In this invention, in the cooling step of cooling the upper surface of the glass gob, the cooling means may directly act upon the molten glass or the glass gob, for example, by bringing a cooling member into contact with the glass gob or may indirectly act upon the glass gob, for example, by blowing cool air to the glass gob. It is noted here that a time period before the pressing step is limited because the lower part of the glass gob is cooled by the lower die preliminarily held around the glass transition point Tg and, if the lower part is excessively cooled so that its viscosity is increased beyond a particular level, press-molding becomes impossible. In order to reserve, within the above-mentioned limited time period, a longer time sufficient to achieve uniform heat distribution in the glass gob in the subsequent heat radiation suppressing step, it is preferable to use the directly-acting cooling means having a high cooling effect and to complete the cooling step in a short time. It is desired that the cooling member is held at a temperature as low as possible within a range such that no quality defect is produced in the glass gob when it is contacted by the cooling member. Furthermore, it is desired that the cooling member is not likely to contaminate the glass gob and has a large heat capacity as well as a high heat conductivity. The cooling member must have a certain level of heat resistance because it is brought into contact with the glass gob, i.e., the molten glass. Specifically, the cooling member is preferably made of a solid metal material. For example, copper is suitable because the heat conductivity is high and the melting point is not lower than 1000xc2x0 C. Although the heat conductivity is not so high, iron is low in cost and easy in machining and can be used without any problem in function. The cooling member may have various structures. Preferably, the cooling member has a circular section so as to avoid the temperature difference in the circumferential direction of the glass gob and is provided with a hole formed at its center to extend in a vertical direction. Preferably, the cooling member continuously carries out a cooling operation (air-cooling or water-cooling) so as to maintain a predetermined low temperature.
In this case, the step of cooling the upper surface of the glass gob may include the step of bringing a heat absorber into contact with the upper surface of the glass, preferably, the step of forcing the heat absorber to a predetermined depth, or the step of forcing the heat absorber at a predetermined contact area to a predetermined depth. Prior to contact with the glass gob, the heat absorber is preferably kept at a predetermined temperature lower than the temperature of the glass gob so as to maintain reproducibility of the cooling conditions.
Preferably, the heat radiation suppressing step of suppressing heat radiation from the glass gob includes the step of making a heat shielding member lower in temperature than the inner part of the glass gob approach the upper part of the glass gob in a non-contact state for a predetermined period or time interval (preferably, 3 to 50 seconds). Preferably, the above-mentioned step is performed by making the heat shielding member having an emissivity of 0.4 or less at least at its surface facing the upper surface of the glass gob approach the upper surface of the glass gob in a non-contact state. By performing this step, the temperature difference between the inner part and the upper surface of the glass gob is preferably suppressed to 100xc2x0 C. or less, preferably 50xc2x0 C. or less, more preferably 30xc2x0 C. or less.
Preferably, a plurality of lower dies are transferred successively to operating positions for the above-mentioned steps to carry out the above-mentioned steps.
Preferably, the lower dies are arranged on a turntable in a circumferential direction and transferred by the rotation of the turntable successively to the operating positions for the above-mentioned steps to carry out the above-mentioned steps.
Preferably, the glass product molded by the above-mentioned method is an optical lens material.
According to this invention, an apparatus for molding a glass product comprises a mold composed of an upper die and a lower die each of which has a molding surface, supplying means for supplying a molten glass as a glass gob onto the molding surface of the lower die, cooling means for cooling an upper surface of the glass gob supplied onto the molding surface of the lower die, heat radiation suppressing means for suppressing heat radiation from the glass gob cooled by the cooling means so that an inner part and an upper part of the glass gob are close in temperature to each other, and mold driving means for making the molding surfaces of the upper and the lower dies approach each other to press the glass gob. When the glass gob whose inner part and the upper surface are made to close in temperature to each other by the heat radiation suppressing means has a viscosity within a range between 103.5 and 106.5 poises (dPaxc2x7s), the mold driving means is activated to press the glass gob.
Preferably, the heat radiation suppressing means comprises a heat shielding member lower in temperature than the inner part of the glass gob and means for making the heat shielding member approach the upper part of the glass gob in a non-contact state.
Preferably, the heat shielding member has an emissivity of 0.4 or less at least at its surface facing the upper surface of the glass gob. More preferably, the heat shielding member comprises a heat insulator material coated with a coating layer having an emissivity of 0.4 or less at least at its surface facing the upper surface of the glass gob.